The synthesis of methanol is commercially achieved through a synthesis gas containing hydrogen (H.sub.2), carbon monoxide (CO), carbon dioxide (CO.sub.2) and small amounts of inert gases such as methane and nitrogen. Carbon oxides react with hydrogen to form methanol according to the following equations: EQU CO+2H.sub.2 .fwdarw.CH.sub.3 OH EQU CO.sub.2 +3H.sub.2 .fwdarw.CH.sub.3 OH+H.sub.2 O
If x, y and z are the molal contents of CO, CO.sub.2 and H.sub.2 respectively in the synthesis gas, the stoichiometric composition of the latter corresponds to the following relationship: EQU z=2x+3y
The optimum composition that is usually aimed at is that which leads to the lowest pressure in the methanol synthesis loop for a given production rate, everything else being equal. Said optimum composition can be either identical to the stoichiometric composition, or very slightly different therefrom, because of kinetic reasons connected to the activity and selectivity of the synthesis catalyst, and also due to differences in solubilities of the various reacting gases in liquid methanol.
In the present technology of methanol production starting from a light hydrocarbon feedstock, ranging from natural gas to naphtha, said feedstock is usually first desulfurized and then steam reformed at moderate pressure, in the range of 15 to 25 atm, and at high temperature, in the range of 850.degree. to 900.degree. C. This endothermic reaction is carried out in refractory tubes, externally heated by a set of burners, and filled with a fixed bed of catalyst made essentially of nickel on a refractory support.
Due to the low carbon/hydrogen ratio of such feedstocks and the minimum steam rate which must be used in steam reforming, the synthesis gas produced by such conventional technology has a composition very different from the stoichiometric composition required for methanol synthesis. Said synthesis gas is then cooled and compressed to the pressure used for the methanol synthesis, which ranges from 50 to 100 atm in the so-called "low pressure" processes, and which may reach 300 atm in the older high pressure processes. The synthesis loop operates then with a very large excess of hydrogen, due to the non-stoichiometric composition of the synthesis gas, which leads to a large purge rate from the synthesis loop, the purge gas being generally used as fuel.
The main drawbacks of the above-described conventional processing scheme, of which some are particularly pronounced when a large capacity unit is considered, that is above 2000 metric tons/day of methanol, can be briefly summarized as: (1) a need to purge large quantities of hydrogen from the synthesis loop limits the capacity of the latter; said capacity would be appreciably larger if the synthesis gas had the stoichiometric composition; (2) the low reforming pressure in the synthesis gas preparation, as well as the high purge rate from the synthesis loop, lead to a poor overall efficiency; (3) the high CO.sub.2 content of the synthesis gas, as well as the non-stoichiometric composition of the latter, require pressurizing at a very high synthesis gas rate; (4) the horsepower and the dimensions of the synthesis gas compressor become excessive for methanol capacities above 3000 tons/day; (5) the cost of the steam reforming heater, which is a very large fraction of the overall plant cost, increases about linearly with capacity, which means that very little gain can be achieved by scaling up to a large single train capacity; (6) the high CO.sub.2 content of the synthesis gas, generally above 10% by volume, produces correlatively important amounts of water in the synthesis loop, thereby increasing the cost of fractioning the methanolwater mixture.
One variation of the above described conventional methanol production scheme consists in adding CO.sub.2 upstream or downstream of the steam reforming step, thereby yielding a synthesis gas with a stoichiometric composition. Said variation is interesting only in those cases where the CO.sub.2 is available at very low cost from a nearby source. Furthermore, said variation does not avoid the other drawbacks mentioned above, and can be used only in very particular circumstances.
In several applications of synthesis gases for other than methanol production, one has to produce a gas having a high CO content, or a low H.sub.2 /CO molal ratio, in the range of 1.5 to 2.5. This is the case for oxo synthesis gas production, pure CO production, and reducing gas production for direct reduction of iron ore for example. As for the case of methanol, it is necessary in the present technology for these cases to operate the steam reforming at high temperature and low pressure; in addition, the synthesis gas produced has a high H.sub.2 /CO ratio, due to the minimum steam/hydrocarbon ratio that must be used in the steam reforming process. This situation is partially improved when an outside source of CO.sub.2 is available, as mentioned above for the case of methanol.
The main purpose of the present invention is precisely to avoid the above mentioned drawbacks, by producing the synthesis gas at high pressure and with a composition adjustable at will, and in particular equal to the stoichiometric composition required for methanol synthesis, thereby reducing appreciably the equipment sizes in the synthesis loop as well as the need for compression of the synthesis gas, even eliminating entirely said compression in some cases.
Besides the above-described conventional steam reforming process for the production of methanol synthesis gas, a so-called "combination process" could be used, whereby the whole feedstock undergoes first a primary steam reforming reaction, and the resulting effluent then undergoes a secondary reforming with oxygen, in a single stage reactor operating adiabatically and packed with a single catalyst bed. Such a process, as described in U.S. Pat. No. 3,264,066, and also mentioned in U.S. Pat. No. 3,388,074, is essentially that widely used in the ammonia industry in which air is replaced by oxygen. Although said combination process allows the use of higher operating pressures in the synthesis gas generation, it does not lead to a final synthesis gas having the stoichiometric composition required for methanol synthesis or a low H.sub.2 /CO ratio, due to the minimum amount of steam that must be used in the primary steam reforming reaction, and for the same reason does not permit a low CO.sub.2 content in said synthesis gas.
In U.S. Pat. No. 3,278,452 a process is described for the production of hydrogen and synthesis gases, in which part of the feedstock undergoes a primary steam reforming reaction, and the effluent therefrom is mixed with the other fraction of the feedstock, and the mixture obtained is passed in a secondary reforming reactor through a succession of conversion zones with oxygen introduced between each until the desired conversion is reached. While this process, which is essentially oriented toward the production of hydrogen and ammonia synthesis gas, may to some extent yield a gas approaching the stoichiometric composition required for methanol synthesis, it still leads to a high CO.sub.2 content in the synthesis gas and it requires a costly multistage reactor to perform the oxygen reforming reaction, and furthermore the injection of oxygen between the successive catalyst beds, operating at very high temperatures, requires the solution of very elaborate technological problems.
In the foregoing process, the need to use a succession of conversion zones, that is a multistage oxygen reforming reactor, arises from the fact that, due to the high concentration of hydrocarbons in the feed to the secondary reformer, the use of the prior art process, where all the oxygen is introduced in a single step reaction, would lead to carbon formation and excessive temperatures in said secondary reformer, as outlined throughout the aforesaid patent. According to the prior art knowledge, the formation of carbon or carbonaceous material is believed to occur outside a certain temperature range of about 600.degree. to 1500.degree. C., and the excessive temperature is attributed to the hypothetical flame temperature reached by assuming instantaneous reaction of all the oxygen present to produce carbon dioxide and steam, with the total heat of reaction being absorbed as sensible heat by the products of the reaction. Accordingly, in the aforesaid process, the amount of oxygen introduced in each catalyst bed is such that the corresponding flame temperature is below the upper limit of 1550.degree. C. which is believed to be reached on the catalyst, and beyond which carbon formation would occur.
Furthermore, it has been reported in the prior art, as outlined in U.S. Pat. No. 3,278,452, that in a single stage oxygen reforming of a hydrocarbon-containing feedstock, the maximum amount of conversion that may be achieved is such that the percentage methane equivalent of the product gas is about one-fifth of that of the feedstock, when the latter is above 25 percent. The expression "percent methane equivalent" as used herein means mole percent of hydrocarbons expressed as methane on a dry basis, e.g. ten mole percent ethane is 20 percent methane equivalent.
It is an object of the present invention to provide a process in which operation of a .single stage secondary oxygen reforming is possible without carbon formation or excessive temperatures, while still achieving a degree of hydrocarbon conversion such that the percentage methane equivalent of the product gas is lower than at least onetenth of that of the inlet feed to said secondary reformer.
Another object of the present invention is to provide a process combining a primary steam reforming with a single stage secondary oxygen reforming, in such a way as to obtain a synthesis gas having essentially the stoichiometric composition required for methanol synthesis, and simultaneously a low CO.sub.2 content.